Organic light-emitting diodes (OLEDs) are a promising technology for flat-panel displays and area illumination lamps. The technology relies upon thin-film layers of organic materials coated upon a substrate. OLED devices generally can have two formats known as small-molecule devices such as disclosed in U.S. Pat. No. 4,476,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic EL element, and a cathode. The organic EL element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), an emissive layer (EL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the EL layer. Tang et al. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989) and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved. However, the materials comprising the organic EL element are sensitive and, in particular, are easily destroyed by moisture and high temperatures (for example greater than 140 degrees C.).
Transparent conductive electrodes typically employ sputter-deposited conductive metal oxides such as indium tin oxide. The described sputter deposited electrode layers, as well as underlying layers, typically are not sufficiently impermeable to environmental contaminants when employed as the transparent top electrode in a top-emitting device, necessitating the use of additional encapsulating overcoat layers or sealed transparent glass covers, thereby exacerbating problems with light trapping and/or increased costs for such devices.
It is well known that OLED materials are subject to degradation in the presence of environmental contaminants, in particular moisture. Organic light-emitting diode (OLED) display devices typically require humidity levels below about 1000 parts per million (ppm) to prevent premature degradation of device performance within a specified operating and/or storage life of the device. Control of the environment to this range of humidity levels within a packaged device is typically achieved, as mentioned above, by encapsulating the device with an encapsulating layer and/or by sealing the device, and/or providing a desiccant within a cover. Desiccants such as, for example, metal oxides, alkaline earth metal oxides, sulfates, metal halides, and perchlorates are used to maintain the humidity level below the above-specified level. See, for example, U.S. Pat. No. 6,226,890 issued May 8, 2001 to Boroson et al. describing desiccant materials for moisture-sensitive electronic devices. Such desiccating materials are typically located around the periphery of an OLED device or over the OLED device itself.
In alternative approaches, an OLED device is encapsulated using thin multilayer coatings of moisture-resistant material. For example, layers of inorganic materials such as metals or metal oxides separated by layers of an organic polymer may be used. Such coatings have been described in, for example, U.S. Pat. Nos. 6,268,695; 6,413,645; 6,522,067, and US Patent Publication No. 2006/0246811, the latter reference hereby incorporated by reference in its entirety.
Such encapsulating layers may be deposited by various techniques, including atomic layer deposition (ALD). One such atomic layer deposition apparatus is further described in WO0182390 to Ghosh et al. entitled “THIN FILM ENCAPSULATION OF ORGANIC LIGHT EMITTING DIODE DEVICES” describes the use of first and second thin film encapsulation layers made of different materials wherein one of the thin-film layers is deposited at 50 nm using atomic layer deposition discussed below. According to this disclosure, a separate protective layer is also employed, e.g., parylene. Such thin multi layer coatings typically attempt to provide a moisture permeation rate of less than 5×10−6 g/m2/day to adequately protect the OLED materials. In contrast, typically polymeric materials have a moisture permeation rate of approximately 0.1 gm/m2/day and cannot adequately protect the OLED materials without additional moisture blocking layers. With the addition of inorganic moisture blocking layers, 0.01 g/m2/day may be achieved and it has been reported that the use of relatively thick polymer smoothing layers with inorganic layers may provide the needed protection. Thick inorganic layers, for example 5 microns or more of ITO or ZnSe, applied by conventional deposition techniques such as sputtering or vacuum evaporation may also provide adequate protection, but thinner conventionally coated layers may only provide protection of 0.01 gm/m2/day. US 2007/0099356 to Park et al. entitled “FLAT PANEL DISPLAY DEVICE AND METHOD OF MANUFACTURING THE SAME” similarly describes a method for thin film encapsulation of flat panel displays using atomic layer deposition.
WO2004105149 TO Carcia et al. entitled “BARRIER FILMS FOR PLASTIC SUBSTRATES FABRICATED BY ATOMIC LAYER DEPOSITION” published Dec. 2, 2004 describes gas permeation barriers that can be deposited on plastic or glass substrates by atomic layer deposition. Atomic Layer Deposition is also known as Atomic Layer Epitaxy (ALE) or atomic layer CVD (ALCVD), and reference to ALD herein is intended to refer to all such equivalent processes. The use of the ALD coatings can reduce permeation by many orders of magnitude at thicknesses of tens of nanometers with low concentrations of coating defects. These thin coatings preserve the flexibility and transparency of the plastic substrate. Such articles are useful in container, electrical, and electronic applications. However, such protective layers also cause additional problems with light trapping in the layers since they may be of lower index than the light-emitting organic layers.
Among the techniques widely used for thin-film deposition is Chemical Vapor Deposition (CVD) that uses chemically reactive molecules that react in a reaction chamber to deposit a desired film on a substrate. Molecular precursors useful for CVD applications comprise elemental (atomic) constituents of the film to be deposited and typically also include additional elements. CVD precursors are volatile molecules that are delivered, in a gaseous phase, to a chamber in order to react at the substrate, forming the thin film thereon. The chemical reaction deposits a thin film with a desired film thickness.
Common to most CVD techniques is the need for application of a well-controlled flux of one or more molecular precursors into the CVD reactor. A substrate is kept at a well-controlled temperature under controlled pressure conditions to promote chemical reaction between these molecular precursors, concurrent with efficient removal of byproducts. Obtaining optimum CVD performance requires the ability to achieve and sustain steady-state conditions of gas flow, temperature, and pressure throughout the process, and the ability to minimize or eliminate transients.
There is interest in utilizing processes for deposition that do not involve the expense associated with vacuum processing. In typical vacuum processing, a large metal chamber and sophisticated vacuum pumping systems are required in order to provide the necessary environment. These items increase the capital cost of systems and preclude the easy use of continuous web based systems.
Various processes for making metal oxide films have been disclosed, both high temperature and low temperature processes, including radio frequency magnetron sputtering or modified reactive planar magnetron sputtering. ALD can be used as a fabrication step for forming a number of types of thin film electronic devices, including semiconductor devices and supporting electronic components such as resistors and capacitors, insulators, bus lines, and other conductive structures. ALD is particularly suited for forming thin layers of metal oxides in the components of electronic devices. General classes of functional materials that can be deposited with ALD include conductors, dielectrics or insulators, and semiconductors.
Advantageously, ALD steps are self-terminating and can deposit precisely one atomic layer when conducted up to or beyond self-termination exposure times. An atomic layer typically ranges from about 0.1 to about 0.5 molecular monolayers, with typical dimensions on the order of no more than a few Angstroms. In ALD, deposition of an atomic layer is the outcome of a chemical reaction between a reactive molecular precursor and the substrate. In each separate ALD reaction-deposition step, the net reaction deposits the desired atomic layer and substantially eliminates “extra” atoms originally included in the molecular precursor. In its most pure form, ALD involves the adsorption and reaction of each of the precursors in the complete absence of the other precursor or precursors of the reaction. In practice in any process it is difficult to avoid some direct reaction of the different precursors leading to a small amount of chemical vapor deposition reaction. The goal of any process claiming to perform ALD is to obtain device performance and attributes commensurate with an ALD process while recognizing that a small amount of CVD reaction can be tolerated.
In ALD applications, typically two molecular precursors are introduced into the ALD reactor in separate stages. For example, a metal precursor molecule, MLx, comprises a metal element, M that is bonded to an atomic or molecular ligand, L. For example, M could be, but would not be restricted to, Al, W, Ta, Si, Zn, etc. The metal precursor reacts with the substrate when the substrate surface is prepared to react directly with the molecular precursor. For example, the substrate surface typically is prepared to include hydrogen-containing ligands, AH or the like, that are reactive with the metal precursor. Sulfur (S), oxygen (O), and Nitrogen (N) are some typical A species. The gaseous precursor molecule effectively reacts with all of the ligands on the substrate surface, resulting in deposition of a single atomic layer of the metal:substrate−AH+MLx→substrate−AMLx-1+HL  (1)where HL is a reaction by-product. During the reaction, the initial surface ligands, AH, are consumed, and the surface becomes covered with AMLx-1 ligands, which cannot further react with metal precursor MLx. Therefore, the reaction self-terminates when all of the initial AH ligands on the surface are replaced with AMLx-1 species. The reaction stage is typically followed by an inert-gas purge stage that eliminates the excess metal precursor and the HL by-product species from the chamber prior to the separate introduction of the other precursor.
A second molecular precursor then is used to restore the surface reactivity of the substrate towards the metal precursor. This is done, for example, by removing the L ligands and re-depositing AH ligands. In this case, the second precursor typically comprises the desired (usually nonmetallic) element A (i.e., O, N, S), and hydrogen (i.e., H2O, NH3, H2S). The next reaction is as follows:substrate−A−ML+AHY→substrate−A−M−AH+HL  (2)
This converts the surface back to its AH-covered state. (Here, for the sake of simplicity, the chemical reactions are not balanced.) The desired additional element, A, is incorporated into the film and the undesired ligands, L, are eliminated as volatile by-products. Once again, the reaction consumes the reactive sites (this time, the L terminated sites) and self-terminates when the reactive sites on the substrate are entirely depleted. The second molecular precursor then is removed from the deposition chamber by flowing inert purge-gas in a second purge stage.
In summary, then, an ALD process requires alternating in sequence the flux of chemicals to the substrate. The representative ALD process, as discussed above, is a cycle having four different operational stages:
1. MLX reaction;
2. MLx purge;
3. AHy reaction; and
4. AHy purge, and then back to stage 1.
This repeated sequence of alternating surface reactions and precursor-removal that restores the substrate surface to its initial reactive state, with intervening purge operations, is a typical ALD deposition cycle. A key feature of ALD operation is the restoration of the substrate to its initial surface chemistry condition. Using this repeated set of steps, a film can be layered onto the substrate in equal metered layers that are all identical in chemical kinetics, deposition per cycle, composition, and thickness.
Self-saturating surface reactions make ALD insensitive to transport non-uniformities, which might otherwise impair surface uniformity, due either to engineering tolerances and the limitations of the flow process or related to surface topography (that is, deposition into three dimensional, high aspect ratio structures). As a general rule, a non-uniform flux of chemicals in a reactive process generally results in different completion times at different areas. However, with ALD, each of the reactions is allowed to complete on the entire substrate surface. Thus, differences in completion kinetics impose no penalty on uniformity. This is because the areas that are first to complete the reaction self-terminate the reaction; other areas are able to continue until the full treated surface undergoes the intended reaction.
Typically, an ALD process deposits about 0.1-0.2 nm of a film in a single ALD cycle (with numbered steps 1 through 4 as listed earlier). A useful and economically feasible cycle time must be achieved in order to provide a uniform film thickness in a range of from about 3 nm to 300 nm for many or most semiconductor applications, and even thicker films for other applications. Industry throughput standards dictate that substrates be processed in 2 minutes to 3 minutes, which means that ALD cycle times must be in a range from about 0.6 seconds to about 6 seconds.
An ALD process must be able to execute this sequencing efficiently and reliably for many cycles in order to allow cost-effective coating of many substrates. In an effort to minimize the time that an ALD reaction needs to reach self-termination, at any given reaction temperature, one approach has been to maximize the flux of chemicals flowing into the ALD reactor, using a so-called “pulsing” process. In the pulsed ALD process, a substrate sits in a chamber and is exposed to the above sequence of gases by allowing a first gas to enter the chamber, followed by a pumping cycle to remove that gas, followed by the introduction of a second gas to the chamber, followed by a pumping cycle to remove the second gas. This sequence can be repeated at any frequency and variations in gas type and/or concentration. The net effect is that the entire chamber experiences a variation in gas composition with time, and thus this type of ALD can be referred to as time dependent ALD. The vast majority of existing ALD processes are time dependent ALD.
In order to maximize the flux of chemicals into the ALD reactor, it is advantageous to introduce the molecular precursors into the ALD reactor with minimum dilution of inert gas and at high pressures. However, these measures work against the need to achieve short cycle times and the rapid removal of these molecular precursors from the ALD reactor. Rapid removal in turn dictates that gas residence time in the ALD reactor be minimized.
Existing ALD approaches have been compromised with the trade-off between the need to shorten reaction times and improve chemical utilization efficiency, and on the other hand, the need to minimize purge-gas residence and chemical removal times. One approach to overcome the inherent limitations of time depended ALD systems is to provide each reactant gas continuously and to move the substrate through each gas in succession. In these systems a relatively constant gas composition exists, but is located to specific areas or spaces of the processing system. Therefore, these systems will be referred to as spatially dependent ALD systems.
For example, U.S. Pat. No. 6,821,563 entitled “GAS DISTRIBUTION SYSTEM FOR CYCLICAL LAYER DEPOSITION” to Yudovsky describes a spatially dependent ALD processing system, under vacuum, having separate gas ports for precursor and purge gases, alternating with vacuum pump ports between each gas port. Each gas port directs its stream of gas vertically downward toward a substrate. The separate gas flows are separated by walls or partitions, with vacuum pumps for evacuating gas on both sides of each gas stream. A lower portion of each partition extends close to the substrate, for example, about 0.5 mm or greater from the substrate surface. In this manner, the lower portions of the partitions are separated from the substrate surface by a distance sufficient to allow the gas streams to flow around the lower portions toward the vacuum ports after the gas streams react with the substrate surface.
A rotary turntable or other transport device is provided for holding one or more substrate wafers. With this arrangement, the substrate is shuttled beneath the different gas streams, effecting ALD deposition thereby. In one embodiment, the substrate is moved in a linear path through a chamber, in which the substrate is passed back and forth a number of times.
Another approach using continuous gas flow spatially dependent ALD is shown in U.S. Pat. No. 4,413,022 entitled “METHOD FOR PERFORMING GROWTH OF COMPOUND THIN FILMS” to Suntola et al. A gas flow array is provided with alternating source gas openings, carrier gas openings, and vacuum exhaust openings. Reciprocating motion of the substrate over the array effects ALD deposition, again, without the need for pulsed operation. In the embodiment of FIGS. 13 and 14, in particular, sequential interactions between a substrate surface and reactive vapors are made by a reciprocating motion of the substrate over a fixed array of source openings. Diffusion barriers are formed by a carrier gas opening between exhaust openings. Suntola et al. state that operation with such an embodiment is possible even at atmospheric pressure, although little or no details of the process, or examples, are provided.
While processes such as those described in the '563 Yudovsky and '022 Suntola et al. patents may avoid some of the difficulties inherent to pulsed gas approaches, these processes have other drawbacks. For example, it would be very difficult to maintain a uniform vacuum at different points in an array and to maintain synchronous gas flow and vacuum at complementary pressures, thus compromising the uniformity of gas flux that is provided to the substrate surface. Neither the gas flow delivery unit of the '563 Yudovsky patent nor the gas flow array of the '022 Suntola et al. patent can be used in closer proximity to the substrate than about 0.5 mm.
U.S. Patent Publication No. 2005/0084610 to Selitser discloses an atmospheric pressure atomic layer chemical vapor deposition process. Selitser states that extraordinary increases in reaction rates are obtained by changing the operating pressure to atmospheric pressure, which will involve orders of magnitude increase in the concentration of reactants, with consequent enhancement of surface reactant rates. The embodiments of Selitser involve separate chambers for each stage of the process, although FIG. 10 shows an embodiment in which chamber walls are removed. A series of separated injectors are spaced around a rotating circular substrate holder track. Each injector incorporates independently operated reactant, purging, and exhaust gas manifolds and controls and acts as one complete mono-layer deposition and reactant purge cycle for each substrate as is passes there under in the process. Little or no specific details of the gas injectors or manifolds are described by Selitser, although it is stated that spacing of the injectors is selected so that cross-contamination from adjacent injectors is prevented by purging gas flows and exhaust manifolds incorporated in each injector.
A spatially dependent ALD process can be accomplished with other apparatus or systems described in more detail in commonly assigned U.S. application Ser. No. 11/392,007 now U.S. Pat. No. 7,413,982, U.S. application Ser. No. 11/392,006 now U.S. Pat. No. 7,456,429, U.S. application Ser. No. 11/620,744, and U.S. application Ser. No. 11/620,740. All these identified applications hereby incorporated by reference in their entirety. These systems attempt to overcome one of the difficult aspects of a spatial ALD system, which is undesired intermixing of the continuously flowing mutually reactive gases. In particular, U.S. application Ser. No. 11/392,007 now U.S. Pat. No. 7,413,982 employs a novel transverse flow pattern to prevent intermixing, while U.S. application Ser. No. 11/620,744 and U.S. application Ser. No. 11/620,740 now U.S. Pat. No. 7,789,961 employ a coating head partially levitated by the pressure of the reactive gases of the process to accomplish improved gas separation.
Despite the usefulness and ease of use of these spatially dependent ALD systems, they continue to be less capable than time dependent ALD systems in terms of separation of the mutually reactive gases. Thus, to render them industrially useful, chemistries must be considered that are more robust to avoid problems associated with gas intermixing.
Thus, there remains still a need to provide a spatially dependent ALD system to make metal-oxide thin film encapsulating films.